Because of the large temperature difference between the superconductor element and the article that is to be connected thereto, i.e. between cryogenic temperature, which may be about −200° C., and ambient temperature, it is necessary to interpose a connection structure between the superconductor element and the article in order to make the temperature transition while minimizing heat flows, and while nevertheless complying with electrical constraints, e.g. due to the high voltage when a cable is involved. This structure includes an electrical bushing made up mainly of a central conductor surrounded by an insulating sheath for conveying electricity from the superconductor cable to an outlet connection at ambient temperature. Over a reasonable length, this structure needs to achieve the temperature transition while ensuring that losses due to heat conduction along the electrical bushing are small, so as to avoid boiling the cryogenic liquid that cools the cable.
Another technical problem that needs to be solved is properly distributing the electric field created by the medium or high voltage of the central conductor of the electrical bushing so as to avoid electric arcs or “breakdowns”.
The central conductor of the bushing is constituted by a cylindrical metal conductor of diameter that is large enough to enable high electric currents to be conveyed. For example, for currents of the order of several thousands of amps, the diameter of the central conductor may be of the order of 50 millimeters (mm) to 70 mm. The voltage is usually high, e.g. 100,000 volts (V), so the sheath surrounding the central conductor must be a good insulator in order to avoid breakdowns occurring, and this must apply over the entire length of the bushing structure, which may be several meters long.
The temperature differences along the bushing lead to a problem of conserving good electrical insulation for the sheath surrounding the central conductor. The materials constituting the conductor (e.g. aluminum or copper) and the sheath (e.g. epoxy) have different thermal conductivities, and that can give rise to stresses at the interface between the conductor and the sheath. Thus, a large amount of heat flowing along the bushing can lead locally to a high temperature gradient between the sheath and the conductor, and that can cause the sheath to crack and thereby destroy its insulating power to a large extent. In addition, the difference between the coefficients of thermal expansion of the materials constituting the conductor and the sheath can lead to mechanical stresses between the sheath and the conductor, and thus to cracking of the sheath.
In order to avoid such cracking occurring, one known technique consists in selecting the materials for constituting the sheath and the central conductor in such a manner that their coefficients of thermal expansion are substantially identical. For example, certain specific aluminum alloy compositions and certain epoxy resins have coefficients of thermal expansion that are very close, with a difference that may be about 5×10−6 per kelvin. That technique serves to limit the risk of cracks forming, but it does not eliminate the risk completely since it is very difficult, if not practically impossible, to find a material that is a good conductor of electricity and another material that is an insulator, both having the same thermal behavior, and in particular both the same coefficient of thermal expansion and the same thermal conductivity. Furthermore, both materials must be capable of withstanding without damage temperature variations over the range about −200° C. to ambient temperature.
European patent application EP 1 283 576 A1 describes an electrical bushing structure of the type comprising:                a central conductor designed to have one of its two ends connected to a superconductor element situated in an enclosure at cryogenic temperature, and its other end connected to an article at ambient temperature;        an electrically insulating sheath surrounding the conductor over substantially the entire length of the conductor; and        a metal tube surrounding the conductor over substantially its entire length and interposed between the insulating sheath and the conductor, the tube being mechanically fastened to the conductor close the end of the conductor that is connected to the superconductor element and not being mechanically fastened to the conductor close to the end of the conductor that is connected to the article at ambient temperature.        
Nevertheless, the tube is made of stainless steel, which is a poor conductor and it does not have any electrical function since it is connected at only one end. All of the current conveyed from the superconductor element to the article at ambient temperature is carried by the conductor.
This results in a loss of working section for conveying current along the bushing and leads to a major risk of heating and of losses via the bushing.